1. Field of the Invention
The present invention relates to a nonaqueous electrolytic solution having high flame retardancy, and more particularly, to a flame-retardant electrolytic solution containing a phosphorus flame retardant having a specific structure and a nonaqueous secondary battery containing the flame-retardant electrolytic solution.
2. Description of the Related Art
In order to achieve size and weight reduction of portable equipment such as notebook computers, video cameras, and mobile phones, the need for secondary batteries having a higher energy density has been increasing. Further, development of batteries having a higher energy density than currently used lead batteries is also awaited for realizing practical use of electric automobiles which emit no air pollutant.
A lithium battery is known for its high energy density. A lithium battery usually contains a nonaqueous electrolytic solution of an electrolyte, such as LiBF.sub.4, LiPF.sub.6, LiClO.sub.4, LiAsF.sub.6, LiCF.sub.3 SO.sub.3, LiAlCl.sub.4 or LiSiF.sub.6, in a mixed solvent comprising a high-dielectric constant solvent, such as propylene carbonate, y-butyrolactone or sulfolane, and a low-viscosity solvent, such as dimethoxyethane, tetrahydrofuran or 1,3-dioxolane.
These solvents used in the nonaqueous electrolytic solution are combustible and incurs a danger of fire in case of a short circuit, etc. A countermeasure against fire is indispensable particularly in a high-energy density battery because the high energy would be released all at once in case of a short circuit.
While fire caused by an overcharge, an overdischarge or an external short circuit has been avoided by an external safety mechanism, such an external safety mechanism does not work on a short circuit inside a battery. It has thus been demanded to develop a battery with safety independent on an external safety mechanism.
To make the solvent flame retardant is known as a means for ensuring the safety independently of the external safety mechanism. Addition of flame-retardant compounds, such as phosphoric esters and halogen compounds, to an electrolytic solution has been proposed to data. For example, Japanese Patent Laid-Open Nos. 184870/92, 283205/94, and 22839/96 teach addition of phosphoric esters, e.g., trimethyl phosphate and tricresyl phosphate. However, these compounds have turned out to cause reduction in charge and discharge efficiency or dendritical precipitation of lithium at the time of charges, making the batteries unsatisfactory for practical use.
Japanese Patent Laid-Open No. 244565/90 proposes use of a small amount of a phosphorus compound, such as a phosphate and a phosphonate, for the purpose of improving the penetrability of an electrolytic solution into a separator, in which no reference is given to the effectiveness of a phosphoric ester compound in improving the flame retardancy of an electrolytic solution.